Kaolin is a naturally occurring, relatively fine, white clay mineral which may be generally described as a hydrated aluminum silicate. After purification and beneficiation, kaolin is widely used as a filler and pigment in various materials, such as rubber and resins, and in various coatings, such as paints and coatings for paper.
The use of kaolin clay in paper coatings serves, for example, to improve brightness, color, gloss, smoothness, opacity, printability and uniformity of appearance of the coated paper. As a filler in paper formulations, kaolin clay is used to extend fiber and reduce cost and to improve opacity, brightness and other desirable characteristics of the paper product.
Calcined kaolin clay is a particular type of kaolin and is often used in large quantities for paper manufacture. Calcined kaolin can be obtained by heating beneficiated kaolin clay at temperatures of at least 550.degree. C. The calcination step dehydroxylates and converts the kaolin to a noncrystalline aluminosilicate phase. The term "dehydroxylates" refers to the removal of structural hydroxide groups from the kaolin as water vapor. Calcined kaolin has improved light scattering characteristics (as compared to the non-calcined kaolin) and, therefore, contributes a high degree of opacity to the coated paper.
Fanselow et al. U.S. Pat. No. 3,586,523 describes examples of calcined kaolin clay.
Calcined kaolin clay pigments (such as the product marketed by Thiele Kaolin Company of Sandersville, Georgia under the trademark KAOCAL) are widely used in the paper industry. Commonly, calcined clay is blended with hydrous kaolin in the paper mills. Typically, these blends are comprised of 10-20 parts by weight calcined kaolin and 90-80 parts by weight hydrous kaolin . During processing, the calcined clay is usually pulverized in a high energy impact mill and then air-classified to remove abrasive particles larger than 325 mesh. The abrasive particles tend to cause scratching problems during coating of a paper.
The resulting pulverized calcined clay product is fluffy and has a low bulk density when compared to a spray dried hydrous kaolin clay. For example, the bulk density of calcined kaolin is about 10-15 lb/ft.sup.3, whereas the bulk density of spray dried hydrous kaolin is about 40-55 lb/ft.sup.3. The fluffy low bulk density calcined clay powder can be difficult to handle with conventional bulk handling systems. Therefore, the powder is shipped dry in bags, sparger cars or as an optimally dispersed slurry, typically at about 50% solids. In addition, the low bulk density of dry calcined clay commonly requires more bags or will occupy a larger volume in sparger cars when compared to a spray dried hydrous clay and is, therefore, more expensive to ship.
In the industry, efforts have been made to improve the bulk density, dusting and powder flow characteristics of calcined clay. For example, in Suitch et al. U.S. Pat. Nos. 5,074,475 and 5,129,593, an aqueous slurry of calcined clay is spray dried to improve bulk density and flowability. However, the spray dried material still creates dust and is difficult to handle in bulk compared to a product agglomerated to a larger size.
Cook et al. U.S. Pat. Nos. 4,561,597 and 4,953,860 increase the bulk density of calcined clay by dry ball milling calcined clay powder, but the flowability of the powder remains poor. Furthermore, the dry ball milling step needs to be followed by pulverizing in a high-energy impact mill to minimize slurry screen residue. However, pulverizing after the dry ball milling step may also produce a product with a lower bulk density.
Dunaway et al. U.S. Pat. No. 5,364,579 pelletizes calcined clay using a pan pelletizer or pin mixer in the presence of 1% ammonia vapor to improve flowability. This process does not use water and, therefore, eliminates the use of an energy intensive drying step. However, the Dunaway et al. patent does not mention any improvement in bulk density or integrity of the pellets compared to the original feed calcined clay.
Crumbley et al. U.S. Pat. No. 5,328,506 discloses a process in which calcined clay is kneaded to form dough-like agglomerates using 1-30% moisture. The claimed advantages of this process are that the product is shipped without drying and is dust free, resists break down to dust during shipping and is capable of being dispersed in water when agitated using conventional make down equipment. However, this process does not use any pressure for agglomeration, and consequently the agglomerated calcined clay product even with 30% hydrous clay does not show an improved bulk density.
Munstennan et al. International Patent Publication No. WO 97/42268 describes a process for producing a dust free, free flowing and high bulk density material using a mixture of water and calcined clay, followed by pelletizing in a disc pelletizer and then drying. The bulk density of the agglomerated product is reported to be about 30 lb/ft.sup.3. The disc pelletizer forms rounded agglomerates by tumbling action, and no external pressure is applied. Consequently, the resulting agglomerated product is expected to be relatively weak.
In view of the foregoing discussion, there is a need in the industry for a process by which calcined clay can be agglomerated to provide a high bulk density, non-dusting, high strength product which can be transported and handled in bulk by conventional systems and which has low screen residue, while still retaining the optical properties which are desirable for paper coating and filling applications.